Google honoured Danish microbiologist Hans Christian Gram with a unique doodle, made by guest artist Mikkel Sommer, on his 166th birth anniversary. The doodle shows a microscope and bacteria in shades of purple as Gram does his experiments.

Born in 1853, the Copenhagen-born devised a staining technique that is now still used to identify, and classify different types of bacteria. He got his M.D. from the University of Copenhagen in 1878, and travelled through Europe studying bacteriology and pharmacology. While working in the lab of German microbiologist Karl Friedländer, he noticed that treating a smear of bacteria with a crystal violet stain, followed by an iodine solution and an organic solvent, revealed differences in the structure and biochemical function of various samples.

Later on, the microbiologist published his findings in a scholarly journal in 1884. Terms like ‘Gram-positive’ and ‘Gram-negative’ were coined after him. Gram-positive bacteria appear purple under a microscope, because their cell walls are so thick that the solvent cannot penetrate them, while Gram-negative bacteria have thinner cell walls that allow the solvent to wash away the stain. Pneumococci, which can result in a lot of diseases, are classified as Gram-positive.

In his publication, Gram had notably included a modest disclaimer: “I have therefore published the method, although I am aware that as yet it is very defective and imperfect; but it is hoped that also in the hands of other investigators it will turn out to be useful.” This simple test, however, proved widely applicable. Gram’s staining method continues to be used today, more than a century later.